Some conventional substrates for mounting semiconductor elements include semiconductor element mounting regions set on the major surfaces (hereinafter, will be referred to as front sides) and a plurality of wiring patterns on the front and back sides.
Generally, substrates for mounting semiconductor elements have been reduced in thickness in response to demand for thin semiconductor devices. Thus the influence of a difference in wiring density between both sides of the substrate considerably increases and causes greater warpage, so that it is difficult to stably apply an adhesive over a semiconductor element mounting region. Further, an unevenly applied adhesive reduces the bonding strength of a semiconductor element and thus reduces the reliability of a semiconductor device.
In order to evenly apply an adhesive, a technique has been proposed in which warpage is suppressed by radially forming wiring patterns from the center of a semiconductor element mounting portion to the outer periphery of a substrate. In the semiconductor element mounting portion, warpage is suppressed by regulating the formation of the wiring patterns on the front side of the substrate and equalizing the wiring densities of the front and back sides of the substrate (Japanese Patent Laid-Open No. 8-153823).
In recent years, there has been an increasing demand for thin semiconductor devices and accordingly, semiconductor elements and substrates for mounting semiconductor elements have been reduced in thickness and adhesives have been reduced in viscosity to improve the stability of assembly. Moreover, in response to demand for semiconductor devices including a larger number of pins with smaller sizes, it has been necessary to form wiring also immediately under semiconductor elements, on substrates for mounting the semiconductor elements.
As substrates for mounting semiconductor elements are reduced in thickness, an amount of deformation such as warpage increases. In the above-mentioned method of radially forming wiring, when a semiconductor element is pressed onto an applied adhesive, the adhesive is likely to leak out of the semiconductor element through a gap between the semiconductor element and a wire and a gap between wires. Further, air is likely to be entrained immediately under the semiconductor element from a surrounding part through the gaps.
When the adhesive leaks out of the semiconductor element, the adhesive becomes thin and uneven immediately under the semiconductor element, causing an insufficient bonding strength. When air is entrained directly under the semiconductor element, the air expands due to a thermal stress applied when the semiconductor device is mounted on a mounting board, so that the semiconductor device may be broken or the reliability may be reduced.